1. Field of the Invention
The present invention relates to a spectroscopic method and relative apparatus for measuring electrode gap distance. In particular, the present invention relates to an in situ spectroscopic method that obtains a gap distance between electrodes without stopping a plasma process, and related apparatus.
2. Description of the Related Art
In a typical plasma etch chamber, the distance between the top electrode and the processed wafer is of critical importance in determining etching rate. FIG. 1 illustrates a simplified setup in a plasma chamber with a top electrode and wafer at the bottom. The plasma chamber is equipped with a top electrode 10 and a bottom electrode 12. RF power supply is coupled to the electrode 10 and the bottom electrode 12 and provides RF power to the space between them. When the RF power supply is on, numerous electric field lines run between the top and bottom to sustain a plasma space formed between them, and a semiconductor wafer 17 on the bottom electrode 12 can be processed as required. The process speed, such as etch rate, is determined by many parameters, such as the plasma density and the excited state of the ions in the plasma space.
Usually the top electrode is made of Si, SiC and other special materials. The top electrode gradually wears of with use over time. Thus, it is inevitable that the gap becomes larger than when it was initially installed. Since the electric field strength (shown by the electric lines 14 in FIG. 1) depends on the distance between electrodes, the plasma density changes accordingly. This results in an observable change of etch rate (ER).
To prevent this issue, it is a routine for an equipment engineer to calibrate and adjust the gap distance after wet cleaning the chamber. However, the calibration methods are usually rough and not very accurate. For example, several clay blocks are placed in the chamber. When the top lid, on which the top electrode is mounted, is closed, these clay blocks are pressed to a certain thickness. The clay blocks are taken out and their thicknesses are measured by a micrometer. The thickness corresponds to the gap distance. After wet cleaning the chamber, if the ER is not normal and the gap distance is questioned, the chamber needs to be reopened to measure the gap distance. This is time-consuming and inaccurate. The manufacturing process has to be interrupted during the procedure for the calibration of gap distance.
An object of the present invention is to in situ measure the gap distance. The object of the invention is achieved by observing the spectrum radiated from the plasma space, by which an accurate gap distance can be determined without opening the top lid.
This invention provides a method for measuring a gap distance between two electrodes. According to the method, a plasma space is formed between the electrodes, across which a DC voltage is coupled. The plasma space has a reactive gas that emits a spectrum of spectral lines. The spectrum is monitored to determine at least one line distance between the spectral lines. Finally, the gap distance can be deduced according to the line distance and a specific rule.
An apparatus having ability of in situ measuring a gap distance between electrodes is also provided. The apparatus comprises a DC voltage source, a plasma chamber, a scanning spectrometer and a data processor. The DC voltage source provides a DC voltage bias coupling across the electrodes. The plasma chamber, within which the electrodes are mounted, forms a plasma space between the electrodes when the DC voltage bias is supplied. The scanning spectrometer monitors spectral lines emitted from the plasma space. The data processor determines the gap distance according to a line distance between the spectral lines and a built-in rule.
The line distance between the spectral lines corresponds to the electric field between the electrode and the electric field, which in turn corresponds to the gap distance. This allows the gap distance can be deduced by the line space.
The major advantage of the present invention is that the procedure for obtaining the gap distance can be executed without opening a top lid of a plasma chamber. Thus, the gap distance can be quickly obtained and the manufacturing process is not interrupted.
Another advantage of the present invention is the high accuracy of the measured gap distance. Since it is very easy to distinguish spectral lines emitted by a specific gas from others, the positions (frequencies or wavelengths) of the spectral lines can be accurately found. An accurate line distance and an accurate gap distance can thus be obtained.